driver core: documentation: make it clear that sysfs is optional

The original text suggested that sysfs is mandatory and always
compiled in the kernel.

Signed-off-by: Lucian Adrian Grijincu <lgrijincu@ixiacom.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>